Significance: Bacteroides fragilis (BF), an obligatory anaerobic bacterium that is normally a gut commensal, can become an opportunistic pathogen and cause serious infections. It is the major Bacteroides species isolated from human anaerobic infections (80%). Metronidazole (MET) is among the most commonly used antibiotics against BF. Unfortunately, MET resistance among BF is becoming more prevalent and MET resistant (MET-R) strains have resulted in amputations, sepsis and death. While MET resistance in BF has been canonically ascribed to the presence of the nim gene (coding for the NIM protein which prevents the formation of the toxic MET molecule), our results as well as other reports indicate otherwise and suggest that MET resistance is multifactorial. Innovation: This project seeks to identify mechanisms that confer MET resistance in BF using an integrated approach combining classical genetics, Next Generation Sequencing technology and molecular manipulation to relate function to specific genes. Our lab has become skilled in using these techniques for BF research and has a strong global network of colleagues who will send us clinical MET-R BF isolates. Research strategy: In Aim 1A we will study transcriptome changes among spontaneous, sequential MET-R mutants. In Aim 1B, we will identify gene(s) responsible for MET resistance in a saturated transposon mutant library. The genes identified as likely candidates for involvement in MET resistance will be targeted for molecular manipulation and either deleted or overexpressed, as appropriate (Aim 1C). In Aim 2A, we will study the clinical MET-R isolates with genomic sequencing and RNA-SEQ based transcriptome analysis with subsequent comparisons with genomes and transcriptomes from other clinical and lab isolates. We will also use an unbiased approach to find MET resistance-associated genes by introducing genomic libraries made from clinical MET-R strains into the MET susceptible lab strain, BF638R. In Aim 2C, we will demonstrate functional associations of these genes by molecular manipulation (as in Aim 1C). In Aim 3 we will focus on nim-mediated MET resistance and determine whether certain nim alleles are more likely to confer the high MICs seen in clinically resistant strains (Aim 3A) and whether nim might be transferred between strains via a conjugative transposon mobile element (Aim 3B). Impact: The proposed work will provide important NGS data for the Bacteroides research community and suggest a new integrated approach with which to study this pathogen. The data regarding nim gene activity as well as involvement of other key genes will be used to develop new rapid diagnostic tests for MET resistance. In addition, identifying the multiple factors in MET resistance will provide guidance t drug developers in designing therapy.